The present invention relates to a release layer, and a composition for forming the same that is particularly well suited to a photoconductor element in a liquid electrophotographic system. More specifically, this invention relates to a release coating for the photoconductor element.
Electrophotography forms the technical basis for various well known imaging processes, including photocopying and some forms of laser printing. The basic electrophotographic process involves placing a uniform electrostatic charge on a photoconductor element, imagewise exposing the photoconductor element to activating electromagnetic radiation, also referred to herein as xe2x80x9clight,xe2x80x9d thereby dissipating the charge in the exposed areas, developing the resulting electrostatic latent image with a toner, and transferring the toner image from the photoconductor element to a final substrate, such as paper, either by direct transfer or via an intermediate transfer material.
The structure of a photoconductor element may be a continuous belt, which is supported and circulated by rollers, or a rotatable drum. The photoconductive layer is generally affixed to an electroconductive support. The surface of the photoconductor is either negatively or positively charged such that when activating electromagnetic radiation strikes the photoconductive layer, charge is conducted through the photoconductor in that region to neutralize or reduce the surface potential in the illuminated region. An optional barrier layer may be used over the photoconductive layer to protect the photoconductive layer and extend the service life of the photoconductive layer. Other layers, such as adhesive or priming layers or charge injection blocking layers, are also used in some photoconductor elements.
Typically, a positively charged toner is attracted to those areas of the photoconductor element which retain a negative charge after the imagewise exposure, thereby forming a toner image that corresponds to the electrostatic latent image. The toner need not be positively charged. Some toners are attracted to the areas of the photoconductor element where the charge has been dissipated. The toner may be either a powdered material comprising a blend of polymer and colored particulates, typically carbon, or a liquid material of finely divided solids dispersed in an insulating liquid. Liquid toners are often preferable because they are capable of giving higher resolution images.
The toner image may be transferred to the substrate or an intermediate carrier by means of heat, pressure, a combination of heat and pressure, or electrostatic assist. A common problem that arises at this stage of electrophotographic imaging is poor transfer from the photoconductor to the receptor. Poor transfer may be manifested by low transfer efficiency and low image resolution. Low transfer efficiency results in images that are light and/or speckled. Low image resolution results in images that are fuzzy. These transfer problems may be alleviated by the use of a release coating.
The release layer is applied over the photoconductive layer or over the barrier layer if a barrier layer is being used. The release layer preferably adheres well to the photoconductive, a barrier layer, or a tie layer. Moreover, the release layer must not significantly interfere with the charge dissipation characteristics of the photoconductor construction.
Typical release coatings known in the electrophotographic arts include silicone polymers such as those disclosed in U.S. Pat. Nos. 4,600,673 and 5,733,698. Conventional siloxane release materials tend to swell significantly in the hydrocarbon solvents that are used as carrier liquids in electrophotography. Swollen polymers generally have reduced strength and are more easily abraded or easily delaminate when applied in an electrophotograhic system.
U.S. Pat. No. 5,652,078 describes another type of release layer that includes a cross-linked siloxane polymer with a bimodal distribution of chain lengths between crosslinks, wherein the siloxane polymer is the reaction product of the components comprising:
A) 35 to 80 parts by weight of a siloxane polymer with a high content of functional groups capable of crosslinking having the formula: 
where each R1 and R3 independently is an alkyl group, aryl group, or alkenyl group, R2 is, independently for each group xe2x80x94SiR2R3Oxe2x80x94 and each group xe2x80x94SiR1R1R2, either an alkyl group, an aryl group, or a functional group capable of cross-linking and at least 3% of R2 are functional groups capable of crosslinking, and x is an integer greater than 0; and
B) greater than 0 and less than or equal to 50 parts by weight of a siloxane polymer with a low content of functional groups capable of crosslinking having the formula 
where each R4 and R6 independently is an alkyl group, aryl group, or alkenyl group, R5 is, independently for each group xe2x80x94SiR5R6Oxe2x80x94 and each group xe2x80x94Si(R4)2R5, either an alkyl group, an aryl group or a functional group capable of cross-linking and no more than 2.5% of R5 are functional groups capable of cross-linking, and y is an integer of at least 50. Optionally, the siloxane polymer can include a cross-linking agent, preferably in an amount from 5 to 30 parts by weight.
What is yet needed is a photoconductor that is capable of withstanding more imaging cycles per photoconductor construction and thus, a more durable release layer is desired. Specifically, the release layer should be mechanically durable to withstand abrasion of the various rollers and scrapers which contact the photoconductor element. The release layer must also be resistant to toner carrier liquids.
One aspect of the present invention provides a photoconductor construction comprising a photoconductor layer, and an electroconductive substrate, and a release layer which displays good release properties, as well as good durability, low peel force, preferably less than about 13 g/2.54 cm, and resistance to toner carrier liquids.
Solvent resistance may be improved by adding fillers to or by cross-linking the polymer. However, highly cross-linked or filled systems tend to have increased surface energy that causes a decreased release performance. The present invention provides a release layer that has increased solvent resistance with minimal sacrifice of release properties.
In one embodiment, a release composition includes a siloxane polymer with a bimodal distribution of chain lengths between crosslinks that is, preferably, the reaction product of a polymer with high functionality, a polymer with low functionality, and a cross-linking agent. However, this polymer could alternatively be the cross-linked product of a single polymer provided the functional groups were spaced appropriately to provide a bimodal distribution of chain lengths between crosslinks. Such a polymer can be synthesized using anionic polymerization methods as are known to those skilled in the art.
As used herein, xe2x80x9cfunctionalityxe2x80x9d and xe2x80x9cfunctional groupsxe2x80x9d is an indication of reactive groups. A polymer with high functionality has more reactive groups than a polymer with low functionality. Some reactive groups would include those groups that undergo free radical reactions, condensation reactions, hydrosilation addition reactions, hydrosilane/silanol reactions, or photoinitiated reactions.
As used herein, xe2x80x9cchain length between crosslinksxe2x80x9d indicates how many monomeric units are in the backbone of the polymer between monomeric units from which branching or cross-linking has occurred. The bimodal distribution of such chain lengths indicates that there are a high number of relatively short chains between crosslinks and a high number of relatively long chains between crosslinks, but only a small number of chains having an intermediate length between crosslinks.
The crosslinking of the siloxanes can be undertaken by any of a variety of methods including free radical reactions, condensation reactions, hydrosilylation addition reactions, hydrosilane/silanol reactions, and photoinitiated reactions relying on the activation of an intermediate to induce subsequent cross-linking.
Thus, one aspect of the present invention provides a photoconductor element including an electroconductive substrate, a photoconductive layer on one surface of the electroconductive substrate, and over the photoconductive layer, a release layer.
Another aspect of the present invention provides an electrophotographic system for producing a multi-colored image including a photoreceptor comprising:
an organic photoconductor having a first major surface and a second major surface; and
a barrier layer on the first major surface of the photoconductor;
a positioner for movably positioning the photoreceptor in order that a given portion of the photoreceptor sequentially advances through a plurality of locations in a single pass;
at least one image-wise exposing device for exposing the photoreceptor with radiation modulated in accordance with an image data for one of a plurality of colors in order to partially discharge the photoreceptor to a first discharge level to produce an image-wise distribution of charges on the photoreceptor corresponding to the image data for the one of a plurality of colors;
at least one applicator to apply a first color liquid toner comprising charged particles of the first color and transparent counter-ions, using an electrode electrically biased to a voltage of between the predetermined charge level and the first discharge level, to the photoreceptor as a function of the image-wise distribution of charges on the photoreceptor to form a first color image, wherein a second substantially uniform predetermined photoreceptor charge level results such that it is lower than the first predetermined charge level but being sufficiently high to subsequently repel liquid toner in areas not subsequently further discharged;
a transferor to transfer at least the first color image and the second color image to a medium to form the multi-colored image; and
a drying element for drying the multi-colored image, wherein at least one of the organic photoconductor and the drying element comprises a release layer.
Preferably, either the photoconductor, the drying element, or both is in the form of an endless belt. Alternatively, the photoconductor can be a drum, preferably an amorphous silicon drum.
In accordance with the present invention, the release layer indicated above includes a silicone polymer which is the reaction product of components comprising:
(a) from zero to about 30 parts by weight of a polymer selected from the group consisting of 
wherein R1, R2, R3, R6, R7, R10, R11, and R12 are each independently selected from an alkyl group, an alkenyl group, an aryl group, and an aralkyl group, such that at least one of R6 and R7 is an alkenyl group,
R4, R5, R8, and R9 are each independently selected from an alkyl group, an aryl group, and an aralkyl group,
l, m, and n are each independently an integer so long as the polymer contains greater than 3 mol % vinyl-containing siloxane groups; a (vinyl siloxy)(siloxy)-modified silica having a vinyl content of at least about 0.4 vinyl equivalent/kg; and a combination thereof;
(b) more than about 20 parts by weight of a polymer having the formula 
wherein R13, R14, R15, R18, R19, R22, R23, and R24 are each independently selected from an alkyl group, an alkenyl group, an aryl group, and an aralkyl group, such that at least two of R3, R14, R15, R18, R19, R22, R23, and R24 is an alkenyl group,
R16, R17, R20, and R21 are each independently selected from an alkyl group, an aryl group, and an aralkyl group,
p, q, and r are each independently integers so long as the polymer has less than 3 mol % vinyl-containing siloxane groups; and
(c) up to about 20 parts by weight of a cross-linking agent of the formula 
wherein R36, R37, R38, R43, R44, and R45 are each independently selected from hydrogen, an alkyl group, an aryl group, and an aralkyl group,
R39, R40, R41, and R42 are each independently selected from hydrogen, an alkyl group (preferably having from 1 to 3 carbon atoms), and an aryl group,
X is O, or a divalent organic linking group, and
s and t are independently integers so long as there are at least two functional groups capable of cross-linking per molecule.
Preferably, the polymer in part (a) is from about 5 to about 20 parts by weight, the amount of polymer in part (b) is from about 30 to about 90 parts by weight, and the amount of the crosslinking agent is from about 5 to about 15 parts by weight. More preferably, the amount of the polymer in part (a) is from about 10 to about 16 parts by weight, the amount of polymer in part (b) is from about 60 to about 80 parts by weight, and the amount of crosslinking agent in part (c) is from about 8 to about 12 parts by weight.
Preferably, the polymer of part (a) contains greater than about 7 mol % vinyl-containing siloxane groups. Preferably, at least one of R6 and R7 is a vinyl-containing group.
Preferably, in the polymer in part (b), at least two of R13, R14 , R15 , R18, R19, R22, R23, R24 is a vinyl-containing group such that the polymer in part (b) preferably has a vinyl content less than about 2 mol % vinyl-containing siloxane groups and, more preferably, has a vinyl content less than about 0.5 mol % vinyl-containing siloxane groups.
In one embodiment, the (vinyl siloxy)(siloxy)-modified silica of part (a) is a vinyl Q resin and, more preferably, has the formula: 
wherein R25, R26, R27, R28 , R29, R30, R31, R32, R33, R34, R35, and R36 are each independently selected from an alkyl group, an alkenyl group, an aryl group, and an aralkyl group, wherein at least one alkenyl group, preferably a vinyl-containing group, is present in each of xe2x80x94OSiR25R26R27; xe2x80x94OSiR28R29R30; xe2x80x94OSiR31R32R33; and xe2x80x94OSiR34R35R36 so long as the polymer has a vinyl content of at least about 0.4 vinyl equivalent/kg. One preferred example is a (methylvinylsiloxy), trimethylsiloxy modified silica.
Yet another aspect of the present invention provides a composition for forming a layer having a low peel force comprising:
(a) from zero to about 30 parts by weight of a polymer selected from the group consisting of 
wherein R1, R2, R3, R6, R7, R10, R11, and R12 are each independently selected from an alkyl group, an alkenyl group, an aryl group, and an aralkyl group, such that at least one of R6 and R7 is an alkenyl group,
R4, R5, R8, and R9 are each independently selected from an alkyl group, an aryl group, and an aralkyl group,
l, m, and n are each independently an integer so long as the polymer contains greater than 3 mol % vinyl-containing siloxane groups; a (vinyl siloxy)(siloxy)-inodified silica having a vinyl content of at least about 0.4 vinyl equivalent/kg; and a combination thereof;
(b) more than about 20 parts by weight of a polymer having the formula 
wherein R13, R14 R15, R18, R19, R22, R23, and R24 are each independently selected an alkyl group, an alkenyl group, an aryl group, and an aralkyl group, such that at least two of R13, R14, R15, R8, R19, R22, R23, and R24 is an alkenyl group,
R16, R17, R20, and R21 are each independently selected from an alkyl group, an aryl group, and an aralkyl group,
p, q, and r are each independently integers so long as the polymer has less than 3 mol % vinyl-containing siloxane groups; and
(c) up to about 20 parts by weight of a cross-linking agent of the formula 
wherein R36, R37, R38, R43, R44, and R45 are each independently selected from hydrogen, an alkyl group, an aryl group, and an aralkyl group,
R39, R40, R41, and R42 are each independently selected from hydrogen, an alkyl group (preferably having from 1 to 3 carbon atoms), and an aryl group,
X is O, or a divalent organic linking group, and
s and t are independently integers so long as there are at least two functional groups capable of cross-linking per molecule. By xe2x80x9clow peel forcexe2x80x9d it is meant that layer formed with the silicone polymer has a value of less than about 13.0 g/2.54 cm, tested per the xe2x80x9cPeel Forcexe2x80x9d test defined in the Examples herein.
As is well understood in this area, substitution is not only tolerated, but is often advisable and substitution is anticipated on the compounds used in the present invention. As a means of simplifying the discussion and recitation of certain terminology used throughout this application, the terms xe2x80x9cgroupxe2x80x9d and xe2x80x9cmoietyxe2x80x9d are used to differentiate between chemical species that allow for substitution or which may be substituted and those which do not so allow or may not be so substituted. Thus, when the term xe2x80x9cgroupxe2x80x9d is used to describe a chemical compound or substituent, the described chemical material includes the basic group and that group with conventional substitution. Where the term xe2x80x9cmoietyxe2x80x9d is used to describe a chemical compound or substituent, only an unsubstituted chemical material is intended to be included. For example, the phrase xe2x80x9calkyl groupxe2x80x9d is intended to include not only pure open-chain and cyclic saturated hydrocarbon alkyl substituents, such as methyl, ethyl, propyl, t-butyl, cyclohexyl, adamantyl, octadecyl, and the like, but also alkyl substituents bearing further substituents known in the art, such as hydroxyl, alkoxy, vinyl, phenyl, halogen atoms (F, Cl, Br, and I), cyano, nitro, carboxyl, etc. On the other hand, the phrase xe2x80x9calkyl moietyxe2x80x9d is limited to the inclusion of only pure open-chain and cyclic saturated hydrocarbon alkyl substituents, such as methyl, ethyl, propyl, t-butyl, cyclohexyl, adamantyl, octadecyl, and the like. Substituents which react with active ingredients, such as very strongly electrophilic or oxidizing substituents, would of course be excluded by the ordinary skilled artisan as not being inert or harmless.
In addition to being used as a release layer for a photoconductor construction, the release polymer of this invention may advantageously be used as a coating on any substrate where there is a desire for good release properties, abrasion resistance, and resistance to liquid toners and similar solvents, materials or solutions. For example, in one preferred embodiment of the present invention, a drying element in an electrophotographic system, as described herein.